Why people support campaigns? To begin with, they will support you, because they simply like it. But they also want to feel like they are getting something in return.
Give them what the campaign promises
If the campaign is a social event, people will simply want to visit it and see what has been promised to them. If the campaign is about planting trees, people will be happy to be there and help, just because they want this to happen. Give them what you promised!
Make them feel as part of the community
It is really motivating for people to feel that they are part of a community. Communicate with them, make them communicate with each other, exchange ideas, knowledge and experience. Ask them to do things together because the campaign is their journey too.
Ask people with particular skills to help
When you have particular skills, it is a real pleasure when you know you can use them and that you can contribute to something you care about. It brings you a satisfaction and a greater personal value. Ask people with skills to help. Show them that this is important for you and for others.
What else can you give people? Think about it.